The present invention relates to a method and a device for spraying powdered to granular bulk material, particularly for the spray-coating of articles, in which the bulk material is fed from a storage container to a spray device by a main stream of propellant gas as a bulk-material/gas mixture.
Pneumatic spray devices for the spray-coating of articles are known from German Pat. No. 1,266,685, which shows in particular a pneumatic conveyor device which, in accordance with the diffuser or Venturi principle, produces a vacuum for drawing the bulk material through a stream of propellant gas. A spray device for spray-coating in an electrostatic field, suitable for the spraying of bulk material, is known from Swiss Pat. No. 429 517 and from U.S. Pat. No. 4,033,506.
Upon the spray coating of flat articles in an electrostatic field, these articles are transported continuously in boxes on a belt past a spray device. For the spray-coating of hollow articles, it may be necessary to clamp them individually in a turning device and to turn them during the spray-coating. Frequently the spray device must be moved into the hollow space of said articles so that the bulk material sufficiently coats all surfaces in the hollow space. The excess bulk material which does not adhere to the articles is drawn by suction out of the box, cleaned and returned in dosaged quantity to the storage container. The suction device must be of very great power so that a sufficient suction effect can be produced throughout the entire volume of the box. The cleaning of the excess bulk material must be very thorough, since even the smallest traces of impurity from the air make the articles unusable. Even after careful cleaning, the excess bulk material may still contain impurities. Therefore, it must be returned in each case only in small metered quantity to the powder containers.
Upon disconnecting the mainstream of propellant gas, as is necessary for instance in order not to spray any bulk material between the individual articles, the conduits remain filled with bulk material. When the stream of propellant gas is again turned on, the bulk material which has remained in the conduits is discharged in a surge from the spray device, after which the latter can again spray in normal manner. This surge of bulk material must not reach the articles to be coated. Therefore the feed of the bulk material must either not be disconnected between the individual articles or upon reconnection, the spray must at first be directed against a screening wall and this wall, after the surge has blown away, then be removed from the spray jet path. This is time-consuming and cumbersome and a relatively large amount of bulk material is lost or must be purified for reuse.